Philadelphia (Ph)- chromosome positive leukemias are relatively common and have a poor prognosis. The Ph-chromosome is generated by a reciprocal t(9;22) translocation, which fuses the ABL and BCR genes from chromosomes 9 and 22. P190 and P210 are BCR/ABL fusion proteins made on the Ph-chromosome, and are the direct cause of leukemia. Mouse models for this type of human leukemia are valuable for studying possible methods of treatment, early stages of the disease, and exploring the signalling pathways which lead to tumorigenesis in vivo. In the previous period of grant support the investigator generated and characterized P210 and P190 BCR/ABL transgenic mice, and, in addition, bcr -/- mice. The current proposal builds on the data and strains previously generated to address specific gaps in our knowledge concerning the pathobiology of and mechanism of transformation by BCR/ABL in vivo. Specifically, Dr. Heistercamp postulates that activity of the ABL/BCR reciprocal translocation product influences the development of leukemia associated with P210. To test this, she will assess biological activity of ABL/BCR in vivo on hematopoiesis and compare leukemogenesis in P210 ABL/BCR + BCR/ABL with that in P210 BCR/ABL transgenic mice. How BCR/ABL causes leukemia is of obvious interest and recent experiments have shown the interaction of a large number of proteins with BCR/ABL in vivo. They intend to test the biological relevance of such interactions to human Ph-positive leukemia in vivo. They will examine if the normal bcr protein influences leukemogenesis and if so, by which mechanism, in BCR/ABL P190 transgenic mice lacking the endogenous bcr gene. They have identified Crkl, an SH2-SH3 adaptor protein, as a substrate for BCR/ABL with clear relevance to Ph- positive leukemia in man: Crkl is specifically tyrosine-phosphorylated only in patient material expressing BCR/ABL. Is crkl required for BCR/ABL mediated tumorigenesis and if so, through which mechanism? This will be studied in crkl -/- and CRKL x BCR/ABL P190 transgenics. Is leukemogenesis caused by BCR/ABL mediated through interaction of signalling molecules such as Grb-2 with phosphorylated tyrosine residue 177 encoded by BCR exon 1? This will be examined in P190 BCR/ABL transgenic mice, in which tyrosine residue 177 of BCR exon 1 has been mutated. They will study if and how interactions with the cytoskeleton, through the actin-binding domain of Abl, are important for the development of BCR/ABL associated leukemia in vivo, in mice transgenic for BCR/ABL P190 which lacks the actin-binding domain of ABL. These experiments will result in the identification of signalling pathways which transduce the transforming signal of BCR/ABL, an important step towards the ultimate goal of developing methods of treatment for Ph-positive leukemias.